PCIe Card Heat Pipe Case Study

Introduction

Aavid, Thermal division of Boyd Corporation, took on the challenging
task of cooling a high power PCIe card housed within a rack-mounted
satellite tracking, telemetry and telecommand (TT&C) unit. This product
functions as a critical bridge between the satellite control center and the
satellite itself.

The Challenge

The PCIe card in question needed to dissipate 40-50W of power,
depending on operating conditions. A customer limitation on additional
active cooling required that Aavid find a passive solution that utilized
the existing fan array in the system.

The primary focus of the cooling solution was to maximize MTBF
of the PCIe card by keeping BGA device temperatures at a minimum.

The Solution

For this level of power dissipation, a heat pipe / radiator fin design was
the most effective choice. Today’s high end computer graphics cards
utilize a similar system to dump their heat; however, most of these are
actively cooled with a small onboard fan. In order to keep the system
passive, Aavid decided to look at a dual slot design where a larger fin
stack would occupy the second slot.

Aavid created a detailed CFD base model which included FPGA, PHY,
DDR, DDS, Omap, Bridge and LTM packages. The PCB thermal properties
were defined by the number of ground and power layers specified by
the customer. Other components in the system, such as the RF and Heat
Sink boards, were modeled as heat dissipating PCB boards. The system
was then analyzed at an ambient temperature of 55°C with an altitude
pressure of 750 hPa and a flow rate of 300LFM.

From the base CFD simulation, Aavid was able to determine flow and temperature maps over the
board, component junction temperatures and case thermals at the above conditions. This base
model was then altered by the following to finalize the thermal solution:

-For components at or above their thermal limit, calculations were done to find the mini
mum required heat sink thermal resistance to bring them into safe operating ranges.
-Designed the thermal solution for all the components exceeding the minimum junction
temperature requirements.
-Optimized the heat sink for DFM, cost and performance.

The Deliverables/Results

A complete thermal report and sample units were provided to the customer for testing. These sample
units, produced at Aavid’s rapid prototyping facility at our headquarters in Laconia, NH, allowed the
customer quickly and effectively validate the thermal solution. After the successful sample tests, Aavid
set forth on a full production run of the PCIe card cooling solution for use in the final product. From
concept to production - and everything in between - Aavid was able to deliver a cost effective and
timely solution for this difficult thermal problem.

-At the above tested conditions (55°C, 750hPa
and 300LFM), all of the components stayed
be low the maximum junction temperature
limits.
-Side baffles were recommended because,
if included, they would prevent air bypass and
improve the junction temperatures further.
-The strategic 3D routing of heat pipes allowed
heat to spread across the surface of the pcb
as well as up into the fins.